Shale gas, an unconventional type of natural gas which is produced from other than usual oilfields or gasfields, is a natural gas which is trapped in an extremely hard layer of rock, that is, a layer of shale, hundreds to thousands of meters below the surface. To extract shale gas, it is necessary to hydraulically fracture the extremely hard shale layer and collect the gas which had been trapped inside the rock layer from deep underground, so higher strength is being sought from steel pipe which is used for production of shale gas.
As the high strength steel pipe for production of shale gas, steel pipe which has a strength equivalent to the API standard 5CT P110 (below, referred to as “P110”) (yield stress YS: 758 to 965 MPa, tensile strength TS: 862 MPa or more) is generally being used. To secure such strength, after pipe manufacturing, the steel pipe as a whole is quenched and tempered. However, in order to cut the cost of shale gas production, demand for electric-resistance welded steel pipe which has high strength and which can be used in a state of being as-formed as-formed into a pipe without heat treatment after pipe manufacturing (omitting the quenching and tempering) has been increasing.
Electric-resistance welding forming (ERW forming) of high strength steel pipe becomes difficult as the tensile strength (TS) becomes higher. Therefore, it is desirable to increase the ratio of the yield strength (YS) and TS, that is, the yield ratio (YS/TS, below referred to as the “YR”) to obtain the targeted YS with a low TS so as to secure the formability. However, if forming high strength steel plate by ERW forming, due to the Bauschinger effect, the yield ratio in the rolling direction (L direction) easily falls. In particular, in steel with dual-phase structure, the Bauschinger effect remarkably appears, so the yield ratio easily falls.
PLT 1 discloses electric-resistance welded steel pipe which utilizes work-hardening to secure a strength equivalent to P110 and which enables elimination of heat treatment after pipe manufacturing. This uses not dual-phase steel, but steel plate of a uniform bainite structure. That is, it discloses to use a cooling rate VC90 which provides a hardness equivalent to a 90% martensite structure as estimated from the C content as an indicator of the hardenability and to control the VC90 to a suitable range and realize a uniform bainite structure to obtain electric-resistance welded steel pipe which has high strength and high yield ratio.
Further, according to PLT 1, in low carbon boron steel, a uniform bainite structure is formed, the Bauschinger effect is small, the YR becomes high, and a YS which satisfies P110 is obtained by a hot rolled steel plate having a low TS. PLT 1 demonstrates that a strength of this standard can be realized.